WO 99/31144 discloses the use of dithiocarbamic esters for regulating the degree of polymerization during the polymerization of vinyl monomers.
DE-A 21 56 453 discloses dialkoxyxanthogen disulphides and their use as regulators during the polymerization of dienes.
The use of dithiocarboxylic esters and of xanthogenic esters for regulating the polymerization of vinyl monomers is described in WO 98/01478. WO 01/42312 discloses the use of dithiocarboxylic esters and in and WO 98/58974 discloses the use of xanthogenic esters.
WO 99/35177 and Tetrahedron Letters 1999, pp. 2435 et seq. also discuss polymerization regulators.
No process has been described for the controlled preparation of polymers based on dienes in the presence of dithiocarboxylic ester, xanthogenic ester, dithiocarbamic ester, giving industrially relevant molar masses Mn>50 000 g/mol with good yields in an acceptable time, but such a process is desirable.
Useful dithiocarboxylic esters/xanthogenic esters/dithiocarbamic esters must in principle be capable of industrial preparation with good purity and with high yield in order to be suitable for industrial use in regulating the degree of polymerization during the polymerization of vinyl monomers and diene monomers.
Processes for preparing dithiocarbamic esters are known and are described by way of example in Houben-Weyl (eds.: K. H. Bückel, J. Falbe, H. Hagemann, M. Hanack, B. Klamann, R. Kreher, H. Kropf, M. Regitz), Thieme Verlag, Stuttgart 1983, 4th edition, Volume E 4, pp. 458-478.
Dithiocarbamic esters may generally be prepared by the following process known from the literature. Dithiocarbamates are prepared by reacting amines with carbon disulphide in the presence of at least equimolar amounts of a base (e.g. potassium hydroxide) in aqueous solution, and after isolation these may be reacted with an organic halogen compound to give dithiocarbamic esters, with elimination of the salt derived from the base and the halide.
WO 99/31144 discloses the use of sodium hydride in organic solvents as a deprotonating reagent.
Dithiocarbamic esters prepared by the synthesis methods of WO 99/31144 have little suitability for industrial use, due to excessively low yields and inadequate product selectivity. The purities obtained are inadequate for industrial use of these dithiocarbamic esters as regulators (i.e. for regulating the degree of polymerization during the polymerization of monomers) unless a purification step is inserted.
The dithiocarbamic esters prepared as described in WO 99/31144 may, where appropriate, be subjected to distillation, recrystallization or chromatography to obtain products sufficiently pure to permit their use as polymerization regulators. High purity of the dithiocarbamic esters is necessary, since the (strong-smelling) by products identified by analysis, some of which are mercaptans, themselves exhibit regulating activity in the polymerization of vinyl monomers and dienes.
Distillation generates high losses, since the dithiocarbamic esters are inherently thermally unstable, and can in turn lead to the by-products mentioned. Purification via crystallization is also associated with significant yield losses, while chromatographic purification on an industrial scale is too expensive.
A process which permits the production of suitable dithiocarbamic esters from inexpensive starting chemicals at good purity would therefore represent a significant technical advance.
The purity of the dithiocarbamic esters prepared by the process of the patent was determined not only by testing for the customary physical data but also by testing their suitability as molecular weight regulators during the emulsion polymerization of chloroprene. For this, chloroprene was polymerized in a standardized process with addition of controlled amounts of dithiocarbamic ester. The resultant elastomer (polychloroprene) was worked up. The solution viscosity of the resultant product was determined from the solid (5% strength solution in toluene) or latex (8.6% strength solution in toluene) with the aid of a Brookfield viscometer at 20° C. The molecular weight Mn (the average molar mass) was determined by means of GPC (gel permeation chromatography), using polystyrene calibration as reference. Chloroprene, which can be polymerized to give polychloroprene, has the following structure:

Familiar regulators for the polymerization of chloroprene, e.g. dodecyl mercaptan or xanthogen disulphides, give products with only modest control of molecular weight. The molecular weights obtained (for example determined using the Mooney viscosity to ISO 289 with no pretreatment) depend on the amount of regulator used, the particle size, the diffusion of the regulator, and also the number of active chains in a micelle in the case of emulsion polymerization. Adjustment of molecular weight via the conversion is not possible here.
The preparation of polychloroprene from chloroprene is known. It is usually carried out as an emulsion polymerization. The emulsion polymerization process proceeds in two stages, the polymerization to give the latex being carried out in the first stage, and the work-up of the latex to give the finished rubber being carried out in the second stage, for example by freeze coagulation. The process requires a product molecular weight of >20 Mooney units (see ISO 289) in order to ensure processability in the above-mentioned process. This corresponds approximately to a number-average molar mass (Mn) of about >100,000 g/mol by GPC (polystyrene calibration).
An object of the present invention is therefore to provide a compound which is suitable for regulating the molecular weight during the polymerization of one or more different monomers, at least one monomer containing a diene group.
Another object of the present invention is to provide a process for preparing this compound, and also to provide a process for polymerizing monomers in the presence of this compound.
Another object of the present invention is to provide polymers, which are prepared by polymerization in the presence of the compound mentioned.